I received a reply in the newsgroup: microsoft.public.outlook.installation. Here it is:
The reason why mesages are downloaded again is because the information on
which messages have been downloaded already is not kept in the PST, but in
the mail profile in the registry. Moving the PST from one machine to the
other cannot synchronize that informaiton. There are several approaches.
One (and the best, in my opinion) is to disable the option of leaving
messages on the server. If you must leave them there, for whatever reason,
then the next best solution is to use a web interface to the server mailbox
and move them to a folder other than the Inbox on the server, if your server
allows you to create alternate folders on the server that way (most do). If
the messages aren't in the Inbox, Outlook won't see them.
Another approach is to create a new PST, make it the delivery location,
allow all the messages to download into that PST, then switch the delivery
location back to the original PST, closing the added one and, thereby,
removing the duplicates all in one go. From that point on, Outlook will
know what is has downloaded already.
A third alternative is to just let all the messages download again and
acquire a duplicate remover. See
http://www.slipstick.com/addins/mail_duplicates.asp . At least one of the
duplicate removers there is free.
--
Brian Tillman